Cameras have used active rangefinders to measure the distance to a remote object. Specifically, a light-emitting device and a light-receiving device are disposed at a given interval. The light-emitting device projects light onto the subject. The reflected light falls on the light-receiving device as a light spot. The distance to the subject is calculated on the principle of triangulation.
In this kind of active rangefinder, the light reflected by the subject falls on the spot light position detector. The displacement of the position of the spot light from a reference point is transformed into two electric currents, which are then converted into corresponding voltage signals by current-to-voltage converter circuits. Subsequently, the ratio of the sum of these two voltage signals to the difference between them is calculated to find the distance.
The level of the output from the spot light position detector is in proportion to the distance of the spot light from the reference point. Also, the output level is proportional to the intensity of the incident extraneous light, i.e., the intensity of light coming from the whole subject. Therefore, if the brightness of the subject is high, head amplifiers acting also as current-to-voltage converters are saturated. This makes it impossible to measure the distance.
Accordingly, the gains of the head amplifiers are lowered to such extent that even if the brightness of the subject is high, the amplifiers are not saturated. Thus, the distance measurement can be made over a wide range of brightness. However, when an object of low brightness is photographed, the decrease in the gain is coupled with a reduction in the depth of focus of the camera lens. This results in a reduction in the focusing accuracy.